Cohort events

@dan-f @jimabramson @mulby Please review

@stroilova FYI
Is there any particular reason you placed this category of events here? I would have put them after forum events, though my logic might not stand up to close examination. (Roughly, I sequenced event categories by how likely a user was to engage in a particular type of activity, multiplied by how interested researchers seem to be in the activity, and moved events for optional features toward the end.)